Side stream removal of impurities in electrolysis systems

ABSTRACT

A side stream subsystem can be used to remove impurity species from the recirculating alkali metal chloride solution in certain electrolysis systems. Silicon and/or aluminum species can be removed via precipitation after introducing an alkali metal hydroxide and magnesium chloride in a side stream line in the subsystem. The invention can allow for a substantial reduction in raw material and capital costs.

TECHNICAL FIELD

The present invention pertains to apparatus and methods for removingimpurities in industrial electrolysis systems. In particular, itpertains to removal of silicon and/or aluminum species in side streamsin chlor-alkali and chlorate electrolysis systems.

BACKGROUND

Industrial electrolysis systems in which brines of various kinds aresubjected to electrolysis in order to produce other useful chemicalproducts have been operating on a large scale for decades. Inparticular, chlor-alkali and chlorate electrolysis systems have beenused to provide much of the chlorine, sodium hydroxide, and chlorateproducts which are subsequently used to prepare other chemicals or usedin the manufacture of various other products.

As a consequence of increasing environmental concerns coupled with ahighly competitive marketplace, modern chlor-alkali and chlorateproducers are forced to look for alternative ways to minimize the amountof solid and liquid effluent produced as well as ways to reduceoperating and capital costs.

A current strategy for reducing the amount of effluent is to useevaporated salt as a source of raw material to make-up the brine to beelectrolyzed instead of the solar or rock salt typically used in thepast. Evaporated salt is a much purer and cleaner source of salt andtypically has amounts of alkali earth metal and other heavy metalcontaminants that are orders of magnitude lower in concentration. Upondissolution of this purer salt, the resulting brine solution quality issuch that the conventional primary treatment process for the brine insuch electrolysis systems can be eliminated.

In chlor-alkali systems, a supply of brine at an appropriateconcentration is supplied to an electrolyzer where it is partiallyelectrolyzed. Weak spent brine from the electrolyzer is thensupplemented with additional make-up salt in a saturator and is thenrecycled back as brine supply for the electrolyzer. However, theconventional secondary treatment process for the brine in such systemsuses a purification subsystem comprising cationic chelating resins,which are not effective in removing certain impurity species such asaluminum and silica. Historically, such impurity species were removedwith the purge of sludges associated with the conventional primarytreatment process. Thus, with the elimination of this primary treatmentprocess, the aluminum and silica impurity species are not effectivelyremoved by the secondary treatment process and consequently they canaccumulate in the recycling brine circuit as make-up salt is continuallyadded thereto.

These accumulating aluminum and silicon species impurities may beremoved by continuously purging an amount of brine from the recyclingbrine in the main recirculation line in the system. The required amountof purge may vary from about 5 to 30% of the flow rate of the brine inthe main recirculation line depending upon the purity of the supply ofevaporated salt. However, the loss of salt associated with purging isgenerally not considered economical. Thus instead, such impurities aretypically removed by treating the full flow of brine in therecirculation line.

Methods are disclosed in the art for removing aluminum and siliconspecies in brine streams. For instance, U.S. Pat. No. 4,073,706 teachesa process for the removal of trace metals from alkali halide brines. Theaddition of controlled amounts of magnesium ions to brine and subsequentprecipitation of magnesium hydroxide removes metal contaminants, andprovides a brine suitable for use in the electrolytic production ofchlorine and alkali metal hydroxide. In the process, the pH can beadjusted by the addition of NaOH.

Also for example, U.S. Pat. No. 6,746,592 discloses a method for thereduction of soluble aluminum species in an evaporated salt alkali metalhalide brine to provide a brine feedstock suitable for use in achlor-alkali membrane cell process. The method comprising treating thebrine with a suitable amount of magnesium salt and sufficient alkalimetal hydroxide to provide an excess alkalinity concentration to effectprecipitation of a magnesium aluminum hydroxide complex.

Further, U.S. Pat. No. 4,274,929 teaches a process for the removal ofsilicates in alkali metal choride containing industrial waste streams toprovide waste brine streams suitable for use in the electrolyticproduction of chlorine and alkali using a diaphragm electrolytic cell.The process involves adding a soluble magnesium compound to alkali metalchloride solution and precipitating the silicates as compounds ofmagnesium. The process includes adjusting the pH of the alkali metalchloride solution to about 11.5 by adding sodium hydroxide, sodiumcarbonate, or mixtures thereof to render the magnesium silicatesinsoluble in the solution.

Although methods such as the above involving magnesium addition andprecipitation are effective in removing aluminum and silicon species,the high levels required for silicon species removal result in poorfiltration. And, much more expensive filteraid in the filteringsubsystems must be used for reasonable filter cycle times. And whilesuch methods have been used in the art to purify brine streams for or inchlor-alkali electrolysis systems, such methods do not appear to havebeen suggested for use in side streams in such systems. Instead, themethods are used or suggested for use in a main line or recirculationline for the brine streams. Further, such methods do not seem employedin chlorate electrolysis systems.

Despite the maturity and sophistication of modern electrolysis systems,there remains a continuing need for reduction of effluent and forreduction in operating and capital costs. The present inventionaddresses these and other needs as discussed below.

SUMMARY

The present invention includes systems and methods for purifying alkalimetal solution in an electrolysis system and particularly for removingsilicon species and also aluminum species from the solution.

More specifically, the electrolysis system is for electrolyzing analkali metal chloride brine (e.g. sodium chloride brine) and comprisesan electrolyzer, a main line, a recirculation line, and a side streamsubsystem. The main line comprises a main stream of purified brine, theelectrolyzer, and a main stream of spent solution in which the mainstream of purified brine is supplied to the inlet of the electrolyzerand the main stream of spent solution is removed from the outlet of theelectrolyzer. The recirculation line is connected to the main line andrecirculates at least a portion of the solution from the main line.

The side stream subsystem comprises a first side stream line with aninlet and an outlet connected to the recirculation line and isconfigured to remove a portion of the solution from the recirculationline at the inlet and return the portion to the recirculation line atthe outlet. In addition, the side stream subsystem comprises a feed forintroducing alkali metal hydroxide into the first side stream line, afirst feed for introducing magnesium chloride into the first side streamline, a residence tank in the first side stream line downstream of thealkali metal hydroxide and the magnesium chloride feeds, and a filter inthe first side stream line downstream of the residence tank for removingprecipitated impurity species. Employing the side stream subsystem ofthe invention can allow for a substantial reduction in raw material andcapital costs.

In an embodiment particularly suitable for removing both silicon andaluminum species, the electrolysis system comprises a second side streamline. For example, the first magnesium chloride feed in the electrolysissystem can be located downstream of the alkali metal hydroxide feed inthe first side stream line. And in addition, the side stream subsystemcomprises a second side stream line and a second feed for introducingmagnesium chloride into the second side stream line. The inlet of thesecond side stream line is connected to the first side stream linebetween the alkali metal hydroxide feed and the first magnesium chloridefeed, and the outlet of the second side stream line is connected to thefirst side stream line between the residence tank and the filter. Thus,in this arrangement, the second side stream line bypasses the residencetank. The side stream subsystem in this embodiment can further comprisea mixing tank between the residence tank and the filter and the outletof the second side stream line can be connected to the mixing tank.

In all the preceding embodiments, the side stream subsystems cancomprise static mixers downstream of each of the alkali metal hydroxide,the first magnesium chloride, and the second magnesium chloride feeds.

The invention can be employed in a chlor-alkali electrolysis system inwhich the electrolyzer is a chlor-alkali electrolyzer, and therecirculation line is the main line and recirculates the solution in themain line from the outlet to the inlet of the chlor-alkali electrolyzer.Such a chlor-alkali electrolysis system can comprise one or morepurification subsystems in the recirculation line for purifying thesolution, and one or more make-up subsystems in the recirculation linefor introducing additional alkali metal chloride and water into thesolution.

In a suitable embodiment of a chlor-alkali electrolysis system, thefirst side stream line can be configured to remove less than about 50%and/or more than about 5% of the solution in the recirculation line.Further, the alkali metal hydroxide and the first magnesium chloridefeeds can be introduced into the first side stream line at the same ordifferent locations.

The invention can also be employed in a chlorate electrolysis system inwhich the electrolyzer is a chlorate electrolyzer, and the systemcomprises a chlorate reactor in the main line to further reactelectrolyzed chlorate solution from the chlorate electrolyzer to moreconcentrated chlorate solution, and a chlorate crystallization subsystemin the main line downstream of the chlorate reactor for crystallizingchlorate from the more concentrated chlorate solution. In the chlorateelectrolysis system, the recirculation line recirculates chloratesolution from the crystallization subsystem to the chlorate reactor.

In a related method, impurity species are removed from an alkali metalsolution in an electrolysis system comprising an electrolyzer, a mainline, and a recirculation line. The method steps include:

-   -   removing a portion of the solution from the recirculation line        into a first side stream,    -   introducing alkali metal hydroxide into the first side stream,    -   introducing magnesium chloride into the first side stream,    -   directing the first side stream to a residence tank after        introducing the alkali metal hydroxide and the magnesium        chloride,    -   allowing the first side stream to reside in the residence tank        for a period of time (e.g. less than about 300 minutes and        particularly between about 60 and 120 minutes),    -   filtering the first side stream after residing in the residence        tank, and    -   returning the solution portion from the first side stream into        the recirculation line.

As mentioned previously, in some embodiments a second side stream can beadvantageously employed. The method steps can then additionally include:

-   -   introducing alkali metal hydroxide into the first side stream        before introducing the magnesium chloride into the first side        stream,    -   removing a side stream portion of the solution from the first        side stream into the second side stream after introducing the        alkali metal hydroxide,    -   introducing magnesium chloride into the second side stream, and    -   returning the side stream portion from the second side stream        into the first side stream.

In this way, the side stream portion of the solution bypasses theresidence tank.

The method is suitable for both chlor-alkali electrolysis systems andfor chlorate electrolysis systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a chlor-alkali electrolysis systemcomprising a first side stream line for the removal of silicon andaluminum species in accordance with the invention.

FIG. 2 shows a schematic view of a chlor-alkali electrolysis systemcomprising first and second side stream lines for the removal of siliconand aluminum species respectively in accordance with the invention.

FIG. 3 shows a schematic view of a chlorate electrolysis systemcomprising a side stream subsystem mainly for the removal of siliconspecies in accordance with the invention.

DETAILED DESCRIPTION

Unless the context requires otherwise, throughout this specification andclaims, the words “comprise”, “comprising” and the like are to beconstrued in an open, inclusive sense. The words “a”, “an”, and the likeare to be considered as meaning at least one and not limited to justone.

In addition, the following definition is intended. In a numericalcontext, the word “about” is to be construed as meaning plus or minus10%.

The present invention represents an improved process and apparatus forremoving silicon and aluminum species impurities from alkali metalsolution, especially in electrolysis systems in which the brine isprepared using purer sources of salt, e.g. evaporated salt.

In the process, brine is treated with magnesium chloride and an alkalimetal hydroxide (e.g. sodium hydroxide) such that a suitable magnesiumconcentration and alkalinity in the resulting brine is obtained. Thiscauses the silicon and aluminum species in the brine to form complexeswith the magnesium, which precipitate out of solution and can then beremoved by filtration. Appropriate values for the magnesiumconcentration and alkalinity are disclosed in the aforementioned priorart that are suitable for both silicon and aluminum species removal(e.g. Mg concentration between about 20 to 60 ppm and sufficient alkalimetal hydroxide to provide an excess alkalinity concentration of between0.05-0.1 g/L alkali metal hydroxide). For effective removal of siliconspecies, a significant residence time for the treated brine is required(e.g. 60 to 120 minutes) in order to fully accomplish complexing andprecipitation. On the other hand, effective removal of aluminum speciescan be accomplished without a significant residence time requirement forthe treated brine.

While conventional electrolysis systems may employ related treatmentprocesses in a main brine line or main recirculation line, in thepresent improved system, treatment is carried out in a side streamconnected to a recirculation line. In this way, satisfactory removal ofthese impurities can be achieved with lower operating and capital costs.

FIG. 1 shows a schematic view of a chlor-alkali electrolysis systemcomprising a first side stream line for the removal of both silicon andaluminum species in accordance with the invention. Chlor-alkalielectrolysis system 1 comprises many of the components and subsystems,in similar configuration, to those found in conventional chlor-alkalielectrolysis systems. The main line in system 1 is itself arecirculation line and comprises membrane electrolyzer 3, dechlorinationsubsystem 5, brine saturator 6, guard filter 8, and ion exchangesubsystem 9 which are interconnected in a loop as shown. In additionthough, system 1 comprises side stream subsystem 2 (indicated by thedashed box in FIG. 1). Specifically, system 1 includes membraneelectrolyzer 3 having inlet 3 a and outlet 3 b. A stream of purifiedbrine is supplied to electrolyzer inlet 3 a from main line/recirculationline 4. A stream of spent brine is removed from electrolyzer outlet 3 bto main line/recirculation line 4. Following electrolysis the spentbrine is directed to dechlorination subsystem 5, which is a purificationsubsystem for removing chlorine from the brine stream. Then, the brinestream is directed to brine saturator 6 where make-up sodium chlorideand demineralized water or sodium chloride brine water is added at saltand water feed 7 (using evaporated salt as a source) to bring up thesalt concentration in the stream to the desired level for electrolysis.A portion of the brine stream is then directed to guard filter 8 wheresolid particulates and precipitates are removed from the stream. Fromthere, the brine is directed via main line/recirculation line 4 to asecondary purification subsystem comprising ion exchange subsystem 9 forensuring very low hardness levels in the purified brine stream. Finally,purified brine from ion exchange subsystem 9 is again available as asupply for electrolyzer 3 thereby completing the recirculation of thebrine.

Side stream subsystem 2 comprises side stream line 10 whose inlet 10 aand outlet 10 b are connected to main line/recirculation line 4. Aportion or fraction of the brine stream in main line/recirculation line4 is removed and directed into side stream line 10 at inlet 10 a. Theportion to be removed depends in part on the purity of the sodiumchloride salt provided at salt and water feed 7 and in part on removalefficiency. Considering that the impurity removal efficiency using theenvisaged treatment process is expected to be about 30 to 90% efficient,the portion of the brine stream in the main line/recirculation line tobe removed would generally be about or slightly higher than the rate ofpurge which would be used if purging was employed to remove theaccumulating impurities instead (namely expected to be about 5-50% ofthe brine in the recirculation line).

An amount of magnesium chloride salt appropriate for the impuritypresent in side stream 10 is then added at magnesium chloride feed 11.As shown in FIG. 1, the side stream is directed to static mixer 13 formixing. Next, an appropriate amount of sodium hydroxide is added toadjust the alkalinity at sodium hydroxide feed 12. The side stream isthen directed to another static mixer 14 for mixing. (Note: while FIG. 1shows an initial addition of MgCl₂ followed by an addition of NaOH, theorder of addition may be reversed or in fact both may be addedsimultaneously.) Afterwards the side stream is directed to residencetank 15 where it resides for less than 300 minutes, particularly about60 to 120 minutes. During this time, magnesium complexes are formed andprecipitate out of the brine solution. The side stream is then pumpedthrough precoat filter 17 by pump 16 to remove solids. An amount ofhydrochloric acid is added as required to readjust the side stream pH athydrochloric acid feed 18. Finally, the treated side stream rejoins mainline/recirculation line 4 and side stream outlet 10 b.

In the embodiment of FIG. 1, both silicon and aluminum species presentin the side stream are removed by this treatment. Because the amount ofbrine being treated in side stream line 10 is only a fraction of thatwhich is conventionally treated in main line/recirculation line 4, thecapital cost of the components in side stream subsystem 2 is much lessthan their conventional equivalents for treating in larger mainline/recirculation line 4. Further, the consumption of raw materialssuch as magnesium chloride and sodium hydroxide is reduced considerably.And, the resulting solids (effluent cake) are also reduced as well.

While the electrolysis system of FIG. 1 typically employs evaporatedsalt as a source of make-up salt, it can also employ solar salt (or thelike) which contains a relatively low amount of magnesium but highsilica. Aluminum species can easily be removed from it but the silicapresent cannot be removed through conventional primary or secondarybrine treatment.

FIG. 2 shows an alternative embodiment of a chlor-alkali electrolysissystem of the invention comprising first and second side stream linesfor the removal of silicon and aluminum species respectively. In FIG. 2,the components common to those in FIG. 1 have been identified with thesame numerals. Here, electrolysis system 20 again comprises manycomponents common to a conventional chlor-alkali electrolysis system,but in addition side stream subsystem 21 has been incorporated forremoving impurities.

Side stream subsystem 21 again comprises first side stream line 10 whichdirects a first side stream to residence tank 15 after adding and mixingtogether appropriate amounts of supplied NaOH and MgCl₂. In additionhowever, side stream subsystem 21 includes second side stream line 22which is connected to first side stream line 10 as shown. Inlet 22 a isconnected to line 10 between sodium hydroxide feed 12 and firstmagnesium chloride feed 11. Outlet 22 b is connected to line 10 betweenresidence tank 15 and precoat filter 17, and in particular is connectedto mixing tank 25 located in line 10. A side stream portion of brine isdirected from first side stream line 10 into second side stream line 22and essentially bypasses residence tank 15. A second magnesium chloridefeed 24 is provided to supply MgCl₂ to side stream line 22 and anotherstatic mixer 26 is provided for mixing thereafter.

In the embodiment of FIG. 2, second side stream line 22 is primarily fortreating the brine to remove aluminum species since a substantialresidence time is not required. And first side stream line 10 isprimarily for treating the brine to remove silicon species. Aftertreatment has been accomplished in both side stream lines, the treatedbrine streams are collected and mixed in mixing tank 25. The remainingcomponents in side stream subsystem 21 are similar to those in subsystem2 in FIG. 1.

The dual side stream configuration of FIG. 2 is advantageous because thealuminum concentration limit for brine supplied to the electrolyzer ismuch lower than the silicon concentration limit Yet the aluminum speciescan be reacted, precipitated, and hence removed much more quickly thanthe silicon species. Thus, different flow rates can be employed in eachstream and the amounts of MgCl₂ added at first and second feeds 11, 24can be adjusted optimally for each function. Overall, the embodiment ofFIG. 2 allows for less consumption of magnesium chloride and NaOH andresults in less effluent cake discharge.

In operating the system shown in FIG. 2, silicon species impuritiesco-precipitated with the Mg hydroxide sludge could get re-dissolved ifthe excess NaOH in side stream line 22 is not properly controlled. Toaddress this potential problem, control of excess NaOH in mixing tank 25is suggested via appropriate flow ratio control of the NaOH added atfeed 12 as shown in the FIG. 2, located upstream of second side streaminlet 22 a. For instance, if the flow ratio is adjusted based on 0.1 g/Lexcess NaOH in mixing tank 25, then less than 0.1 g/L excess NaOH couldalways be maintained in first side stream 10 between inlet 22 a andmixing tank 25 (for Si removal) and greater than 0.1 g/L excess NaOHcould always be maintained in second side stream 22 (for Al removal).

FIG. 3 shows a schematic view of a chlorate electrolysis systemcomprising a side stream subsystem in accordance with the invention.Again, note that in FIG. 3, components similar in function to those inFIG. 1 have been identified with the same numerals. Many of thecomponents and their configuration in chlorate electrolysis system 30are similar to those found in conventional chlorate electrolysissystems. Here, the main line in system 30 is more complex and insequence comprises salt and water feed 7, brine saturator 6, guardfilter 8, ion exchange subsystem 9, brine line 35, chlorate reactor 33,line 37, chlorate electrolyzer 32, line 38, chlorate reactor 33 again,chlorate crystallization subsystem 34, and line 39 which areinterconnected as shown. Brine for electrolysis is prepared in brinesaturator 6. A suitable source of salt (e.g. evaporated salt) and asupply of demineralised water is provided at salt and water feed 7. Fromthere, brine is directed via brine line 35 to guard filter 8, then toion exchange subsystem 9 and finally to chlorate reactor 33 where it ismixed with the product from chlorate electrolyzer 32 to maintain thesalt content in the electrolyzer feed. Chlorate reactor 33 directs anelectrolyte solution for electrolysis comprising both chlorate and brineto chlorate electrolyzer 32 via line 37. And electrolyzed chloratesolution from chlorate electrolyzer 32 is directed back to chloratereactor 33 via line 38.

Concentrated product chlorate solution from chlorate reactor 33 isdirected to chlorate crystallization subsystem 34 where chlorate productis crystallized out from the more concentrated chlorate solution andremoved at 39. The leftover solution after crystallizing is recirculatedback to chlorate reactor 33 via recirculation line 36.

Over time, impurities can accumulate in recirculation line 36. To removethese, chlorate electrolysis system 30 has been provided with sidestream subsystem 31 connected in parallel to recirculation line 36.Here, the side stream subsystem is primarily for removal of silicaspecies but will also remove aluminum species. As shown in FIG. 3, aportion of solution is removed from recirculation line 36 at side streamline inlet 10 a. The portion of solution is directed to side streamsubsystem 31 for removing silica and aluminum impurity species (again,indicated by the dashed box). As shown in FIG. 3, the components andtheir configuration in side stream subsystem 31 can be similar to thosein side stream subsystem 2 of FIG. 1. Alternatively, subsystem 31 mayemploy a configuration similar to that shown in FIG. 2.

As in the preceding chlor-alkali electrolysis system embodiments,subsystem 31 may serve to remove both silica and aluminum species. It isprimarily expected to be used for silica removal however. The allowablelimit for aluminum for chlorate electrolyzers is typically an order ofmagnitude higher than that for chlor-alkali electrolyzers. And if brinewas prepared from evaporated salt, removal of aluminum may not berequired. However, in the event that solar salt were employed, aluminumimpurity can be precipitated and filtered out in the form of aluminummagnesium hydroxide complexes using conventional primary brinetreatment.

Advantages of the invention include potential simplification of theelectrolysis systems with a reduction in capital costs and operatingcosts. For instance, the conventional requirement for a precoat filterin the main brine stream may be eliminated. Instead a much smallerprecoat filter may be employed in a side stream line and a guard filterin the main brine stream which reduces the consumption of costlycellulose and effluent cake discharge significantly. Further, a muchsmaller residence tank and pump may be employed. Further, use of theinvention might be expected to provide for a reduction in consumption ofmagnesium chloride salt and sodium hydroxide (e.g. from 50 to 90% in achlor-alkali electrolysis system when compared to conventional systemswhich treat the main brine stream). In addition, the HCl added to thefiltered brine for pH control is reduced as a result of reducing the useof caustic.

The following Examples have been included to illustrate certain aspectsof the invention but should not be construed as limiting in any way.

EXAMPLES

A series of experiments was conducted to determine exemplary removalresults for silica and aluminum species from a typical brine solution asa function of temperature, residence time, and amounts of MgCl₂ and NaOHused.

In all the following, the test brine used was 25% w/w NaCl. Carbonateswere first removed from the test brine by reducing the pH with HClovernight and then returning the pH to neutral with NaOH. Then, aluminumand silica species were added (in the form of commercially availableacidified standard solution) so as to bring the concentrations up to 0.1mg/kg Al and 5 mg/kg SiO₂ respectively, which is typical for anevaporated salt brine solution.

For each test in the series, an amount of MgCl₂ was first added to asample of test brine to obtain a desired concentration of Mg.Thereafter, an amount of NaOH was added to obtain a desired causticconcentration (expressed below as “excess NaOH” and which is in excessto that used to bring the sample to neutral pH prior to adding thealuminum and silica species). The sample was mixed and then allowed toreact for a selected residence time. Finally, the sample contents werefiltered using a ˜1 micron syringe filter and the filtrate was analysedfor residual aluminum and silica content.

In a first set of tests, results were determined at differenttemperatures (from 15 to 35° C.) and amounts of Mg added (from 10 to 40mg/kg). The residence time used here was always 240 minutes. Table 1shows the residual amounts of Al and SiO₂ in mg/kg and also expressesthe latter in terms of % removed.

TABLE 1 Different temperature and Mg addition Mg Excess Final Final SiO₂added NaOH Temp Al SiO₂ removed (mg/kg) (g/L) (° C.) (mg/kg) (mg/kg) (%)10 0.088 15 <0.01 4.55 9% 10 0.088 25 <0.01 4.10 18% 10 0.088 35 <0.013.65 27% 20 0.098 15 0.02 3.70 26% 20 0.098 23 <0.01 3.27 35% 20 0.09835 <0.01 3.00 40% 40 0.107 15 <0.01 2.78 44% 40 0.104 25 <0.01 2.20 56%40 0.109 35 <0.01 1.80 64%

In a second set of tests, results were determined at different residencetimes (from 60 to 240 minutes). Here, constant amounts of 40 mg/kg Mgand 0.1 g/L excess NaOH were added and a constant temperature of 60° C.was used. Table 2 again shows the residual amounts of Al and SiO₂ inmg/kg determined.

TABLE 2 Different residence time Residence Final Final SiO₂ time Al SiO₂removed (minutes) (mg/kg) (mg/kg) (%) 60 <0.01 4.00 20% 120 <0.01 3.0040% 240 <0.01 1.65 67%

In a third set of tests, results were determined for differing amountsof Mg (from 20 to 60 mg/kg) and excess NaOH (from 0.05 to 0.2 g/L)added. Here, a constant temperature of 60° C. and a constant residencetime of 120 minutes were used. Table 3 again shows the residual amountsof Al and SiO₂ in mg/kg determined.

TABLE 3 Different Mg and NaOH addition Excess Mg Final Final SiO₂ NaOHadded Al SiO₂ removed (g/L) (mg/kg) (mg/kg) (mg/kg) (%) 0.05 20 0.013.15 37% 0.1 20 0.01 3.80 24% 0.2 20 0.01 4.35 13% 0.05 40 <0.01 2.7545% 0.1 40 0.01 3.25 35% 0.2 40 <0.01 3.70 26% 0.05 60 <0.01 2.35 53%0.1 60 <0.01 2.80 44% 0.2 60 <0.01 3.65 27%

Exemplary results when using a two side stream process as depicted inFIG. 2 were determined in a fourth set of tests. In each test here, twosamples of test brine were treated separately. A common temperature of60° C. and a common 0.1 g/L excess NaOH were used in both cases. (Inthis set of tests, the excess NaOH was added to both samples before theMg was added.) The first of the two samples however was treated as mightbe done in the first side stream line while the second sample wastreated as might be done in the second side stream line. Here then,either 40 or 60 mg/kg of Mg was added (as indicated below) to the firstsample, which was then allowed to react over a residence time of 120minutes. After this residence period ended, 2 mg/kg of Mg was added tothe second sample. Varied amounts of the two samples (either 1:1 or 2:1by volume for the first sample:second sample) were then mixed togetherimmediately, were filtered as above, and analysed for aluminum andsilica content as above. Table 4 shows the residual amounts of Al andSiO₂ in mg/kg determined for this set of tests.

TABLE 4 Results for two side streams Mg Volume added to 1^(st) ratioFinal Final SiO₂ sample first:second Al SiO₂ removed (mg/kg) sample(mg/kg) (mg/kg) (%) 40 1:1 <0.01 3.55 29% 60 2:1 <0.01 3.10 37%

The above series of tests demonstrates that the method is expected to beeffective in removing aluminum and silica species from typical brines.Over the temperatures tested, silica removal was noticeably improved athigher temperatures while aluminum was always removed and thus notaffected. Further, silica removal increased with longer residence timewhile aluminum again was always removed. Further still, the greater theamount of added Mg, the greater the amount of silica removed, again withno noticeable effect on aluminum removed. And it appears that removal ofaluminum and silica is practical using a two side stream line process,without overly compromising removal efficiency significantly.

In a further illustrative test, silica removal was attempted on a brinesample without providing excess NaOH during the residence period.Specifically, 40 mg/kg Mg was added to a test brine sample at atemperature of 60° C. and allowed to react over a residence time of 120minutes without adding excess NaOH during the residence period. At theend of the residence period, 0.1 g/L NaOH was added but after mixing,the sample was filtered and analysed as above. Only 6% of the silica wasremoved in this example, indicating that excess NaOH is required duringthe residence period for this process to be effective.

All of the above U.S. patents, U.S. patent applications, foreignpatents, foreign patent applications and non-patent publicationsreferred to in this specification, are incorporated herein by referencein their entirety.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art without departing from thespirit and scope of the present disclosure, particularly in light of theforegoing teachings. Such modifications are to be considered within thepurview and scope of the claims appended hereto.

1. An electrolysis system for electrolyzing an alkali metal chloridebrine, the system comprising: an electrolyzer; a main line comprising amain stream of purified brine, the electrolyzer, and a main stream ofspent solution wherein the main stream of purified brine is supplied tothe inlet of the electrolyzer and the main stream of spent solution isremoved from the outlet of the electrolyzer; a recirculation lineconnected to the main line that recirculates at least a portion of thesolution from the main line; and a side stream subsystem comprising: afirst side stream line with an inlet and an outlet connected to therecirculation line and configured to remove a portion of the solutionfrom the recirculation line at the inlet and return the portion to therecirculation line at the outlet; a feed for introducing alkali metalhydroxide into the first side stream line; a first feed for introducingmagnesium chloride into the first side stream line; a residence tank inthe first side stream line downstream of the alkali metal hydroxide andthe magnesium chloride feeds; and a filter in the first side stream linedownstream of the residence tank for removing precipitated impurityspecies.
 2. The electrolysis system of claim 1 wherein the alkali metalis sodium.
 3. The electrolysis system of claim 1 wherein the firstmagnesium chloride feed is located downstream of the alkali metalhydroxide feed in the first side stream line and the side streamsubsystem comprises: a second side stream line wherein the inlet of thesecond side stream line is connected to the first side stream linebetween the alkali metal hydroxide feed and the first magnesium chloridefeed, and the outlet of the second side stream line is connected to thefirst side stream line between the residence tank and the filter,whereby the second side stream line bypasses the residence tank; and asecond feed for introducing magnesium chloride into the second sidestream line.
 4. The electrolysis system of claim 3 wherein the sidestream subsystem comprises a mixing tank between the residence tank andthe filter and the outlet of the second side stream line is connected tothe mixing tank.
 5. The electrolysis system of claim 3 wherein the sidestream subsystem comprises a static mixer downstream of each of thealkali metal hydroxide, the first magnesium chloride, and the secondmagnesium chloride feeds.
 6. The electrolysis system of claim 1 whereinthe electrolysis system is a chlor-alkali electrolysis system, theelectrolyzer is a chlor-alkali electrolyzer, and the recirculation lineis the main line and recirculates the solution in the main line from theoutlet to the inlet of the chlor-alkali electrolyzer.
 7. Thechlor-alkali electrolysis system of claim 6 comprising: at least onepurification subsystem in the recirculation line for purifying thesolution; and at least one make-up subsystem in the recirculation linefor introducing additional alkali metal chloride and water into thesolution.
 8. The chlor-alkali electrolysis system of claim 6 wherein thefirst side stream line is configured to remove less than about 50% ofthe solution in the recirculation line.
 9. The chlor-alkali electrolysissystem of claim 8 wherein the first side stream line is configured toremove more than about 5% of the solution in the recirculation line. 10.The chlor-alkali electrolysis system of claim 6 wherein the side streamsubsystem comprises at least one static mixer downstream of the alkalimetal hydroxide and the first magnesium chloride feeds.
 11. Thechlor-alkali electrolysis system of claim 6 wherein the alkali metalhydroxide and the first magnesium chloride feeds are introduced into thefirst side stream line at the same location.
 12. The electrolysis systemof claim 1 wherein: the electrolysis system is a chlorate electrolysissystem; the electrolyzer is a chlorate electrolyzer; the systemcomprises: a chlorate reactor in the main line to further reactelectrolyzed chlorate solution from the chlorate electrolyzer to moreconcentrated chlorate solution; and a chlorate crystallization subsystemin the main line downstream of the chlorate reactor for crystallizingchlorate from the more concentrated chlorate solution; and therecirculation line recirculates chlorate solution from thecrystallization subsystem to the chlorate reactor.
 13. A method forremoving impurity species from an alkali metal solution in anelectrolysis system, the electrolysis system comprising an electrolyzer;a main line comprising a main stream of purified brine, theelectrolyzer, and a main stream of spent solution wherein the mainstream of purified brine is supplied to the inlet of the electrolyzerand the main stream of spent solution is removed from the outlet of theelectrolyzer; and a recirculation line connected to the main line thatrecirculates at least a portion of the solution from the main line; themethod comprising: removing a portion of the solution from therecirculation line into a first side stream; introducing alkali metalhydroxide into the first side stream; introducing magnesium chlorideinto the first side stream; directing the first side stream to aresidence tank after introducing the alkali metal hydroxide and themagnesium chloride; allowing the first side stream to reside in theresidence tank for a period of time; filtering the first side streamafter residing in the residence tank; and returning the solution portionfrom the first side stream into the recirculation line.
 14. The methodof claim 13 wherein the impurity species removed comprises a siliconspecies.
 15. The method of claim 13 wherein the impurity species removedcomprises an aluminum species.
 16. The method of claim 13 wherein thealkali metal is sodium.
 17. The method of claim 13 comprising:introducing alkali metal hydroxide into the first side stream beforeintroducing the magnesium chloride into the first side stream; removinga side stream portion of the solution from the first side stream into asecond side stream after introducing the alkali metal hydroxide;introducing magnesium chloride into the second side stream; andreturning the side stream portion from the second side stream into thefirst side stream, whereby the side stream portion of the solutionbypasses the residence tank.
 18. The method of claim 13 wherein theelectrolysis system is a chlor-alkali electrolysis system, theelectrolyzer is a chlor-alkali electrolyzer, and the recirculation lineis the main line and recirculates the solution in the main line from theoutlet to the inlet of the chlor-alkali electrolyzer.
 19. The method ofclaim 18 comprising removing less than about 50% of the solution fromthe recirculation line into the first side stream.
 20. The method ofclaim 19 comprising removing more than about 5% of the solution from therecirculation line into the first side stream.
 21. The method of claim13 wherein the electrolysis system is a chlorate electrolysis system;the electrolyzer is a chlorate electrolyzer; the system comprises: achlorate reactor in the main line to further react electrolyzed chloratesolution from the chlorate electrolyzer to more concentrated chloratesolution; and a chlorate crystallization subsystem in the main linedownstream of the chlorate reactor for crystallizing chlorate from themore concentrated chlorate solution; and the recirculation linerecirculates chlorate solution from the crystallization subsystem to thechlorate reactor.
 22. The method of claim 13 wherein the period of timeis less than about 300 minutes.
 23. The method of claim 22 wherein theperiod of time ranges between about 60 and 120 minutes.